In A Wrinkle in Time, what does the dark thing represent?

The Dark
Thing or Black Thing in
represents evil of all types, and it takes over

planets. Planets which have been fully enveloped by the Dark Thing are called dark
planets, and
the children see the planet Uriel, which is dark, and then
Earth, which is partly dark. The
darkness over Earth is being fought by good
people, including Jesus, Gandhi, Leonardo da Vinci,
and Shakespeare. When the
children and the Mrs. Ws are looking at Earth with the Medium, Calvin
asks
what the Dark Thing is: We know that its evil, but what is it?

Mrs....